This R21 proposal is in direct response to PA-13-145, entitled Development of Animal Models and Related Biological Materials for Research. The goal of this proposal is to develop two new animal models to fill major unmet needs in a broad array of fields, including immunology, virology, and stem cell medicine. The surface of all nucleated cells are covered in MHC class I molecules that present peptides derived from proteins made in the cell. T cells scan the surface of a cell, and engage only cells in which their T cell receptor has affinity for a specific MHC-peptide complex. Once the T cell engages, the outcome is highly dependent on the cell presenting the antigen and the local context. Different cell types can result in different T cell responses; for example some cells may direct CD4+ T cell to differentiate into an effector, whereas presentation by other cells may direct them to become regulatory. How a T cell interacts with a particular cell type is one of the most important questions in immunology. The answer can tell us why some cell types are more prone to autoimmune responses, and why other cell types, such as cancer cells, may be less prone to T cell killing. While we know how T cells react with some cell types, there are many cell types whose interaction with T cells has never been studied. For example, we do not know how T cells interact with many types of stem cells. We hypothesize that if green florescent protein (GFP) can be utilized as a model antigen, than T cell interactions with numerous cell types, which were not previously studied including many types of stem cells, can be investigated. The objective of Aim 1 is to generate mice in which the T cells recognize an epitope of GFP presented in MHC class I or II. The outcome of Aim 1 would establish an antigen-specific T cell model that targets a commonly used, easily monitored, florescent reporter. Since GFP-specific CD8+ T cells can kill GFP-expressing cells, we hypothesize that GFP-specific CD8+ T cells can be used to selectively deplete almost any cell population in a mouse, if the cell type of interest s expressing GFP. We have already obtained preliminary data showing that we can use GFP-specific T cells to kill GFP expressing cells in the spleen, pancreas, gut, and heart. The objective of Aim 2 is to compare T cell mediated cell ablation to diphtheria toxin (DT)-mediated depletion, and to determine how effectively we can use the GFP-specific T cells to kill cells in the brain. The outcome of these studies will establish a new method of cell depletion that can be used with the 100s of cell type-specific GFP expressing mice that already exist, and it is potentially less inflammatory than DT-mediated depletion. The abundance of GFP-based reagents and animal models makes the GFP-specific T cell mice relevant for numerous labs doing a wide variety of research including, but not limited to: (i) understanding the antigen presenting capacity of specific cell types, (ii) modeling autoimmune destruction, (iii) characterization of pathogen-T cell interactions, and for (iv) identifying the function of specific cells in vivo.